Field effect transistor with narrow bandgap source and drain regions and method of fabrication

ABSTRACT

A transistor having a narrow bandgap semiconductor source/drain region is described. The transistor includes a gate electrode formed on a gate dielectric layer formed on a silicon layer. A pair of source/drain regions are formed on opposite sides of the gate electrode wherein said pair of source/drain regions comprise a narrow bandgap semiconductor film formed in the silicon layer on opposite sides of the gate electrode.

RELATED APPLICATIONS

This is continuation of pending U.S. patent application Ser. No.12/343,400 filed on Dec. 23, 2008, which is a divisional of U.S. patentapplication Ser. No. 11/064,996 filed on Feb. 23, 2005, now issued asU.S. Pat. No. 7,518,196.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of field effect transistorsand more particularly to a field effect transistor having a pair ofsource/drain regions formed from a narrow bandgap semiconductor film andits method of fabrication.

2. Discussion of Related Art

Integrated circuits, such as microprocessors, digital signal processors,and memory devices are made up of literally millions of transistorscoupled together into functional circuits. An example of a conventionalmetal oxide semiconductor field effect transistor (MOSFET) 100 isillustrated in FIG. 1. Transistor 100 includes a gate electrode 102formed on a gate dielectric layer 104 which in turn is formed on amonocrystalline silicon substrate. A pair of sidewall spacers 108 arethen formed along laterally opposite sidewalls of the gate electrode102. A pair of source/drain regions 110 are then formed along oppositesides of the gate electrode 102 as shown in FIG. 1. The source and drainregions comprise heavily doped portions of the silicon substrate 106.Typically, a silicide layer 112, such as titanium silicide or nickelsilicide, is used to couple contacts 120 formed in a interlayerdielectric 140 to the source and drain regions 110. Silicide regions 112are generally formed by alloying a metal, such as titanium, nickel orcobalt with the silicon substrate 106 to form the metal silicide.Additionally, contacts 120 are generally formed from a relatively highresistance film such as tungsten which can be conformally deposited sothat it fills contact opening formed in the dielectric layer 140.

The dimensions of transistor 100 are continually being scaled down inorder to increase packing density and thereby increase the computationalpower of the fabricated integrated circuits. Unfortunately, astransistor 100 is continually scaled down, the external resistance ofthe device (Rext) is increased degrading device performance, such as itsdrive current. Presently, the problem of increased Rext is solved byhigh active doping of the source and drain region and fully silicidingthe source and drain regions. High active doping of the source and drainregions can decrease the electron mobility in the source and drainregions. Fully siliciding the source and drain regions results in aschkotty barrier transistors resulting in ambipolar conduction.Additionally, formation of silicide films by alloying a metal and thesemiconductor substrate together can increase the thermal budget of thedevice which can decrease device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a cross-sectional view of a standard MOSnonplanar field effect transistor.

FIG. 2A and 2B are illustrations of cross-sectional views of nonplanarfield effect transistors having a pair of source/drain regions formedfrom a narrow bandgap semiconductor in accordance with embodiments ofthe present invention.

FIGS. 3A-3F illustrate a method of fabricating a field effect transistorhaving a pair of source/drain regions comprising a narrow bandgapsemiconductor in accordance with embodiments of the present invention.

FIG. 4 is a plot showing the electron mobility verses sheet carrierconcentration for indium antimonide (InSb).

FIG. 5 is an illustration of a nonplanar transistor having a pair ofsource/drains formed from a narrow bandgap semiconductor.

FIG. 6 illustrates a system in accordance with one embodiment.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention is a field effect transistor having a pair ofsource/drain regions formed from a narrow bandgap semiconductor film. Inthe following description numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Inother instances, well known semiconductor processing techniques andequipment have not been set forth in particular detail in order to notunnecessarily obscure the present invention.

Embodiments of the present invention include a field effect transistorhaving a pair of source/drain regions formed from a narrow energybandgap (e.g., less than 0.75 eV) semiconductor film. In an embodimentof the present invention, the silicon substrate adjacent to the gateelectrode is etched out and refilled with a selective epitaxial regrowthof a narrow bandgap high mobility compound semiconductor material. Byforming the source and drain regions from a narrow bandgap highconductivity semiconductor, such as indium antimonide (InSb), indiumarsenide (InAs), indium gallium arsenide (In_(1−x)Ga_(x)As(x>50%)) andindium phosphide (InP), a significant reduction in the parasitic seriesresistance in extremely scaled sub-50 nanometer MOSFETs can be obtained.Additionally, in embodiments of the present invention, non-alloyed ohmiccontacts are used to make contact to the narrow bandgap semiconductorfilm which results in a very low thermal budget in the front end processthereby improving device performance. Additionally, in embodiments ofthe present invention, the transistor is formed in a thin silicon layerof silicon-on-insulator (SOI) substrate in order to decrease or removethe possibility of increased junction leakage current. Source and drainregions formed from a narrow bandgap semiconductor material can be usedto decrease the parasitic resistance in both planar and nonplanardevices.

FIG. 2A is an example of a metal oxide semiconductor field effecttransistor (MOSFET) 200 in accordance with an embodiment of the presentinvention. Transistor 200 has a gate electrode 202 formed on a gatedielectric layer 204 formed on a silicon layer 206. A pair of sidewallspacers 208 are formed along laterally opposite sidewalls of gateelectrode 202 as shown in FIG. 2A. Transistor 200 has a pair ofsource/drain regions 210 formed on opposite sides of gate electrode 202.A channel region 206 is located in the silicon layer between the sourceand drain regions 210.

In an embodiment of the present invention, the source and drain regions210, or a portion of the source and drain regions 210, are formed from alow or narrow bandgap energy semiconductor film 212. In an embodiment ofthe present invention the narrow bandgap semiconductor is an epitaxialor single crystalline film. In an embodiment of the present invention,the source and drain regions 210 are formed from a narrow bandgapsemiconductor 212 having an energy bandgap of less than 0.75 eV and inembodiments less than 0.36 eV. In an embodiment of the presentinvention, the narrow bandgap semiconductor film 212 is a compoundsemiconductor film having a bandgap between 0.75 eV and 0.18 eV. In anembodiment of the present invention, the source and drain regions 210include a epitaxial narrow bandgap, high mobility compound semiconductormaterial 212, such as but not limited to indium antimonide (InSb),indium arsenide (InAs), indium gallium arsenide(In_(1−x)Ga_(x)As(x>50%)) and indium phosphide (InP). In an embodimentof the present invention, the source and drain region 210 are formedfrom a compound semiconductor 212 having bulk mobility between10,000-80,000 μ (cm²V⁻¹s⁻¹). An advantage of using a narrow bandgapsemiconductor in the source and drain regions, is that they have lowsheet resistivities due to their inherently high mobilities and superiorelectron transport properties as compared to silicon. FIG. 4 is a plotwhich illustrates the electron mobility in InSb vs. the carrierconcentration. The higher mobility results in a low sheet resistance(R_(sd)). Another advantage of using a narrow bandgap semiconductor 212with very high mobility is that it provides a lower band offset, Ø_(b),and a lower effective electron mass (m*) which results in a lowering ofthe contact resistivity, ρc, which results in a lower contactresistance, R_(c), compared to conventional n+ implanted silicon sourceand drain regions with nickel silicide contacts.

In an embodiment of the present invention, transistor 200 has raisedsource and drain regions. Raised source and drain regions are formed byhaving the narrow bandgap semiconductor film 212 extend above the topsurface 201 of the silicon film upon which the gate dielectric layer 204is formed. In an embodiment of the present invention, the narrow bandgapsemiconductor film 212 of the source and drain regions 210 extends atleast 200 Å above the silicon surface 201 and generally between 200-300Åabove the surface 201. The dielectric sidewall spacers 208 isolate theraised portion of the source and drain regions 210 from the gateelectrode 202. As such, the spacers 208 are formed to a width sufficientenough to isolate the narrow bandgap semiconductor films from the gateelectrode 202 as well as wide enough to reduce the parasitic Millercapacitance which can develop between the raised source and drainregions and the gate electrode 202. Additionally, spacers 208 should notbe formed too wide so that current traversal path is increased andpacking density decreased. In an embodiment of the present invention,the sidewall spacers are formed from an insulating film, such as siliconnitride or silicon oxide, having a width of between 100-500 Å.

In an embodiment of the present invention, as illustrated in FIG. 2A,the narrow bandgap semiconductor 212 laterally extends completelybeneath spacer 208 and slightly extends beneath or undercuts the gatedielectric/gate electrode 204/202. In an embodiment of the presentinvention, the narrow bandgap semiconductor 212 extends beneath theoutside edges of the gate electrode 210 by approximately 50-70 Å.

Additionally, when forming an n type field effect transistor (FET) wherethe majority carriers are electrons, the narrow bandgap semiconductor isdoped to an n type conductivity and when forming a p type field effecttransistor where the majority carriers are holes, the narrow bandgapsemiconductor 212 is doped to a p type conductivity. A narrow bandgapsemiconductor 212, such as indium antimonide (InSb), indium arsenide(InAs), indium gallium arsenide (In_(1−x)Ga_(x)As(x>50%)) and indiumphosphide (InP), can be doped to an n type conductivity with, forexample, tellurium (Te), silicon (Si)and sulfur (S) and doped to a ptype conductivity with, for example, carbon (C), cadmium (Cd), zinc (Zn)and chromium (Cr).

In an embodiment of the present invention, the source and drain regionscan include a pair of source/drain extensions or tip regions 214 asillustrated in FIG. 2B. Source/drain extensions are a portion of thesource/drain regions 210 and are formed by doping the silicon layer andextend beneath the gate electrode and spacers. The extensions 214 aredoped to a p type conductivity for a p type device and to an n typeconductivity for a n type device. If the source/drain extensions 214 areprovided, then the narrow bandgap semiconductor portion 212 of thesource and drain regions 210, need not extend beneath the outside edgesof the gate electrode because the laterally tip regions 214 extend thesource and drain regions 210 beneath the spacers 208 and gate electrode202 as shown in FIG. 2B. In an embodiment of the present invention, thenarrow bandgap semiconductor 212 extends only slightly beneath thesidewall spacers 208 where it contacts the source/drain extensions 214.In yet another embodiment of the present invention, the narrow bandgapsemiconductor 212 contacts the source/drain extensions 214 in alignmentwith the outside edges of the sidewall spacers 208.

In an embodiment of the present invention, transistor 200 is formed in asilicon layer which is part of a monocrystalline silicon substrate. Whentransistor 200 is formed in a monocrystalline silicon substrate, thechannel region 206 is formed in a doped region of the monocrystallinesilicon substrate and the narrow bandgap semiconductor film 210 isformed in recesses formed in the monocrystalline silicon substrate. Whena transistor is formed in a monocrystalline silicon substrate, thetransistor is sometimes referred to as “a bulk” transistor.Unfortunately, forming the source and drain regions 210 from a lowbandgap semiconductor, a bulk device can cause an increase the junctionleakage current. Accordingly, in an embodiment of the present invention,in order to reduce the junction leakage current, transistor 200 isformed in a thin epitaxial silicon film 220 of a silicon-on-insulator(SOI) substrate 224. A silicon-on-insulator (SOI) substrate 224 includesa thin silicon film 220 formed on a buried oxide layer 222 which in turnis formed on a monocrystalline silicon substrate 226. In an embodimentof the present invention, the narrow bandgap semiconductor portions 212of the source and drain regions 210 are formed in recesses formed in theepitaxial silicon film 220 of the SOI substrate 226. In an embodiment ofthe present invention, a small portion 230 of the epitaxial silicon film220 remains between the buried oxide layer and the bottom of the narrowbandgap semiconductor 212 so that an epitaxial narrow bandgapsemiconductor film can be selectively grown on the epitaxial siliconfilm 220. In an embodiment of the present invention, the portion 230 ofthe epitaxial silicon film remaining between the buried oxide layer andthe narrow bandgap semiconductor film 212 is between 10-100 Åthick.

Additionally, in an embodiment of the present invention, an interlayerdielectric 240, such as but not limited to a silicon dioxide film or alow k dielectric, such as a fluorine doped silicon dioxide (SiOF), acarbon doped oxide (CDO) or a zeolite dielectric, is formed over andaround transistor 200 as shown in FIGS. 2A and 2B. The interlayerdielectric layer 240 isolates transistor 200 from the upper levels ofmetallization 260 used to interconnect the various transistor 200 formedon substrate 224 into function circuits, such as microprocessors,digital signal processors and memory devices. Metal contacts 250 areformed through the interlayer dielectric and directly contact the narrowbandgap semiconductor material 212 of the source and drain regions 210to provide electrical connection between the first level ofmetallization and the source and drain regions 210 as illustrated inFIG. 2A. In an embodiment of the present invention, the metal contacts250 form a low resistance non-alloyed ohmic contact with a narrowbandgap semiconductor 210. In an embodiment of the present invention,metal contacts 250 comprise a lower titanium adhesion layer 252 and abulk gold (Au) layer 254. In an embodiment of the present invention,there is no silicide or alloyed films formed between the narrow bandgapsemiconductor film 210 and the contact 250. The use of a non-alloyedmetal contact 250 results in a very low thermal budget in the front endof the process flow and its associated benefits.

It is to be appreciated that the present invention is not limited toplanar devices and can be used in nonplanar devices, such as tri-gatetransistors and dual gate transistors. FIG. 5 illustrates a tri-gatetransistor 500 having source and drain regions comprising a narrowbandgap semiconductor film 212. In a nonplanar device, instead offorming the gate dielectric layer 204 on a single surface of a siliconfilm, the silicon film is first patterned into a silicon body having atop surface opposite a bottom surface formed on buried oxide and a pairof laterally sidewalls as shown in FIG. 5. The gate dielectric layer 204is then formed on the top surface and sidewalls of the silicon body asshown in FIG. 5. A gate electrode is then formed on the gate dielectriclayer on the top surface of a silicon body and directly adjacent to thegate dielectric layer 204 on the sidewalls of the semiconductor body sothat the gate electrode 202 surrounds the channel region of the siliconbody on three sides. In a FINFET or dual gate device, the gate electrode202 can be isolated from the silicon body by a thick dielectric layer(not shown) so that the gate only controls two sides of the body. Theportion of the silicon body on opposite sides of the gate electrode usedto form the source and drain regions can then be partially etched awayso that the narrow bandgap semiconductor film 212 can be regrown to formthe source and drain regions 210.

FIGS. 3A-3F illustrate a method of forming a field effect transistorhaving source and drain regions formed from a narrow bandgapsemiconductor film in accordance with embodiments of the presentinvention. The fabrication of a transistor in accordance with thepresent invention begins with a substrate. In an embodiment of thepresent invention, the substrate is a monocrystalline silicon substrate,for example, when forming a bulk semiconductor device. In an embodimentof the present invention, the monocrystalline silicon substrate mayinclude an epitaxial silicon film formed on the monocrystallinesubstrate as is well known in the art. In yet another embodiment of thepresent invention, the substrate is a silicon-on-insulator (SOI), suchas substrate 300 as illustrated in FIG. 3A. In an embodiment of thepresent invention, the silicon-on-insulator substrate 300 includes anepitaxial silicon film 302 formed on a buried oxide layer 304 which inturn is formed on a monocrystalline silicon substrate 306. In anembodiment of the present invention, the buried oxide layer has athickness between 200-2000 Å. Additionally, in an embodiment of thepresent invention, the epitaxial silicon film or layer 302 has athickness of between 10-400 Å.

Isolation regions, not shown, are typically formed in the silicon film302 or substrate adjacent to the device fabrication area to isolate thefabricated transistor from adjacent transistors. The epitaxial siliconfilm 302 is then doped to a conductivity type and concentration desiredfor the channel region of the device. In an embodiment of the presentinvention, when fabricating a p type transistor, the silicon film can bedoped to an n type conductivity and when fabricating an n type devicethe silicon film 302 can be doped to a p type conductivity. Typically,the semiconductor film 302 will be doped to an n type or p typeconductivity with a concentration level between 1×10¹⁶-1×10¹⁹ atoms/cm³.In an embodiment of the present invention, the silicon film 302 is leftundoped or intrinsic in order to fabricate a transistor with an undopedor intrinsic channel region.

Next, as shown in FIG. 3B, a gate dielectric layer 308 is formed on theepitaxial silicon film 302 and a gate electrode 310 formed on the gatedielectric layer 308. The gate dielectric layer can be a deposited orgrown dielectric. In an embodiment of the present invention, the gatedielectric layer is a silicon dioxide or a silicon oxynitride dielectricfilm grown with a wet/dry oxidation process. In an embodiment of thepresent invention, the silicon oxide film is grown to a thicknessbetween 5-15 Å. In an embodiment of the present invention, the gatedielectric layer is a deposited dielectric, such as but not limited to ahigh dielectric constant film (high k), such as a metal oxidedielectric, such as tantalum pentaoxide, titanium oxide, hafnium oxide,zirconium oxide, aluminum oxide, and various silicates or other high kdielectric, such lead zirconium titanate (PZT) and barium strontiumtitanate (BST). A high dielectric constant film can be formed by anywell known technique, such as but not limited to chemical vapordeposition (CVD) and atomic layer deposition (ALD).

The gate electrode 310 can be formed by any well known technique, suchas by blanket depositing a gate electrode material over substrate 300and then utilizing well known photolithography and etching techniques topattern the gate electrode material into a gate electrode 310. In anembodiment of the present invention, the gate electrode 310 has a pairof laterally opposite sidewalls separated by a distance which definesthe gate length (Lg) of the transistor. In an embodiment of the presentinvention, gate electrode 310 has a gate length (Lg) of less than 50nanometers. The gate electrode 310 may be formed from any suitable gateelectrode material. In an embodiment of the present invention, the gateelectrode material may comprise a polycrystalline silicon. In anembodiment of the present invention, the electrode material comprises apolycrystalline silicon germanium alloy. In yet another embodiment ofthe present invention, the gate electrode material may comprise a metalfilm, such as tungsten, tantalum and their nitrides. In an embodiment ofthe present invention, the gate electrode has a thickness or heightbetween 200-3000 Å.

It is to be appreciated that when forming a nonplanar transistor, suchas a tri-gate or dual gate transistor, epitaxial silicon film 302 wouldfirst be patterned into a silicon body having a top surface opposite theburied oxide layer 304 and a pair of laterally opposite sidewalls as iswell known in the art. When forming a tri-gate device, the gatedielectric layer would be formed on the top surface and sidewalls of thefabricated semiconductor body and then the gate electrode formed on andadjacent to the gate dielectric layers so that the gate electrodesurrounds the semiconductor body on three sides as is well known in theart. When forming a dual gate or FINFET device, a thick insulating layermay be left on the top surface of a semiconductor body to isolate thegate electrode from the top surface of a semiconductor body so that thegate electrode controls only the two sidewalls of the body as is wellknown in the art.

Next, after the formation of gate dielectric layer 308 and gateelectrode 310, source/drain extensions 309 may be formed insemiconductor film 302 on opposite sides of gate electrode 310 as shownin FIG. 3B, if desired. Source/drain extensions 309 can be formed by,for example, ion implanting dopants into the silicon layer 302. The gateelectrode 310 can act as a mask to prevent the doping of a channelregion 320 during the ion implantation process resulting in theformation of source/drain extensions 309 which are formed in alignmentwith the outside edges of gate electrode 310. A subsequent anneal can beused to cause the dopants to slightly diffusion beneath the outsideedges of the gate electrode 310 as shown in FIG. 3B. Then gate electrode310 is a polysilicon gate electrode the source/drain extension implantprocess can be used to dope the polysilicon film to reduce itsresistance. The source/drain extensions 309 can be used, for example,when the narrow bandgap semiconductor film for the source and drainregions is not extended beneath the gate electrode. When fabricating atransistor where the narrow bandgap semiconductor laterally extendsbeneath the gate electrode 310, the source/drain extension formationprocess may not be necessary. Omitting the source/drain extensionprocess can help reduce the thermal budget of the front-end process andthereby improve device performance.

Next, as shown in FIG. 3C, a pair of thin sidewall spacers 312 areformed adjacent to the laterally opposite sidewalls of gate electrode310. Sidewall spacers 312 can be formed by blanket depositing aconformal insulating film, such as silicon nitride, silicon oxynitride,or silicon oxide or a combination thereof over substrate 300 and gateelectrode 310 and then anisotropically etching back the dielectric filmto form spacers 312. The thickness of the deposited dielectric filmdetermines the thickness of the spacers 312. In an embodiment of thepresent invention, the spacers 312 are formed to a thickness between100-500 Å. In an embodiment of the present invention, the spacers 312are formed from a silicon nitride film formed by a hot wall low pressurechemical vapor deposition process (LPCVD).

Next, a pair of recesses 316 are formed in silicon film 302 on oppositesides of gate electrode 308 as shown in FIG. 3D. In an embodiment of thepresent invention, recesses 316 extend beneath the outside edge of gateelectrode 310 and gate dielectric layer 308 as shown in FIG. 3D. In anembodiment of the present invention, recesses 316 extend betweenapproximately 50-70 Å beneath the outside edges of the gate electrode310. In an embodiment of the present invention, the recesses are formedto a depth of approximately 5-350 Å beneath the surface 301 of siliconfilm 302 upon which the gate dielectric layer 308 is formed. In anembodiment of the present invention, a portion of the silicon film 302remains in recess 316 above buried oxide layer 304 so that narrowbandgap semiconductor film can be subsequently selectively epitaxiallygrown thereon. Any well known and suitable etching technique can beutilized to form recesses 316. In an embodiment of the presentinvention, a selective etch process is used to form recesses 316. In aselective etch process, an etchant is used which only etches siliconfilms, such as epitaxial silicon layer 302 and not insulating films suchas spacers 310, gate dielectric 308 and isolations regions. (If gateelectrode 310 is formed from polycrystalline silicon, an insulating hardmask can be used to form gate electrode 310 then left on during theetching of recesses 316 to protect the silicon electrode from beingetched). In an embodiment of the present invention, an isotropic etchingprocesses, such as a wet etch, is used to form recesses 316 so that therecess 316 can laterally undercut spacers 312 and gate electrode 310. Inan embodiment of the present invention, recesses 316 are formed with awet etch comprising an ammonium hydroxide or asymmetric or symmetrictetra (alkyl) ammonium hydroxides (alkyl=methyl, ethyl, propyl,isopropyl, butyl, tert-butyl; symmetric meaning all 4 alkyl groups arethe same).

Next, as shown in FIG. 3E, recesses 316 are filled with a narrow bandgapsemiconductor film, such as but not limited to indium antimonide (InSb),indium arsenide (InAs), indium gallium arsenide(In_(1−x)Ga_(x)As(x>50%)) and indium phosphide (InP). In an embodimentof the present invention, the narrow bandgap semiconductor 318 isdeposited to a thickness to fill recess 318 and in another embodiment ofthe present invention, is deposited to a thickness sufficient to extendabove the top surface 301 of silicon film 302 in order to form raised orelevated source and drain regions. In an embodiment of the presentinvention, the narrow bandgap semiconductor film extends at least 200 Åabove top surface 301 of silicon film 302 and in an embodiment of thepresent invention, extends between 200-500 Å.

Any well known and suitable technique can be used to deposit narrowbandgap semiconductor film 318. In an embodiment of the presentinvention, the narrow bandgap semiconductor film 318 is grown with anepitaxial process, such as but not limited to, molecular beam epitaxy(MBE), metal organic chemical vapor deposition (MOCVD), and low pressurechemical vapor deposition (LPCVD).

In embodiment of the present invention, the narrow bandgap semiconductorfilm 318 is insitu doped (i.e., doped while it is deposited) to an ntype or p type conductivity. Semiconductor film 318 can be doped to an ntype conductivity utilizing dopant atoms, such as silicon (Si) ortellurium (Te) and can be doped to a p type conductivity atomsutilizing, for example, carbon (C), cadmium (Cd), zinc (Zn) and chromium(Cr). In an embodiment of the present invention, the narrow bandgapsemiconductor film has a melting temperature between 550-500° C. so thatit can be deposited at a relatively low temperature, such as less than500° C., and still enable dopant atoms to substitute into the narrowbandgap semiconductor lattice without requiring an activation anneal.Eliminating an activation anneal reduces the thermal budget of theprocess used to fabricate the transistor.

In an alternative embodiment of the present invention, the narrowbandgap semiconductor film 318 can be deposited undoped and thensubsequently doped by, for example, ion implantation or solid sourcediffusion.

In an embodiment of the present invention, the narrow bandgapsemiconductor film 318 is selectively grown into recesses 316. In aselective deposition process, the narrow bandgap semiconductor formsonly on silicon locations, such as epitaxial silicon film in recesses316 and not on insulating or dielectric layers, such as sidewall spacers312 and isolation regions. If gate electrode 310 is formed from apolycrystalline silicon or silicon germanium alloy, then an insulatinghard mask can be used during the patterning of the gate electrode andleft on during the growth of the narrow bandgap semiconductor 318 inorder to prevent the formation of a narrow bandgap semiconductor film onthe gate electrode. The epitaxial silicon film 302 in the lower parts ofrecesses 316 provide a single crystalline lattice from which anepitaxial narrow bandgap semiconductor film can be grown.

In an embodiment of the present invention, an InSb epitaxial siliconfilm 318 is grown in recess 316 utilizing a MOCVD process utilizing anequimolar gaseous mixture of volatized trimethylindium and stibane(SbH₃) with about 5% each by mass in a carrier gas such as argon. In anembodiment of the present invention, an indium antimonide (InSb)epitaxial film is grown utilizing molecular beam epitaxial (MBE)utilizing a solid source.

In embodiment of the present invention, non-alloyed ohmic contacts 340are used to electrically couple the source and drain region to the upperlevels of metallization 350. Accordingly, first an interlayer dielectric(ILD) 330, such as but not limited to silicon dioxide (SiO₂), fluorinedoped silicon dioxide (SiOF) and carbon doped oxide (CDO), is blanketdeposited over substrate 300 including gate electrode 310, spacers 312and narrow bandgap semiconductor 318. The interlayer dielectric 330 isformed to a thickness sufficient to isolate the gate electrode 310 andnarrow bandgap semiconductor 318 from the subsequently formed upperlevel of metallization 350. In an embodiment of the present invention,the interlayer dielectric is formed to a thickness between 500 Å-2000 Å.It is to be appreciated that an interlayer dielectric need notnecessarily be a single layer dielectric and can be a compositedielectric layer formed from multiple insulating layers. Afterdepositing the interlayer dielectric, the interlayer dielectric may beplanarized to provide a planar top surface.

Next, contact openings can then be formed through interlayer dielectric330 utilizing well known photolithography and etching techniques atlocations where contacts 340 are desired to contact the narrow bandgapsemiconductor 318. Conductive contacts 340 are then formed into theopenings and make direct contact with the narrow bandgap semiconductorfilm 318. In an embodiment of the present invention, contacts 340 areformed by blanket depositing a conductive contact film by, for example,chemical vapor deposition (CVD) or atomic layer deposition (ALD) intothe openings and onto the top surface of ILD 330. The conductive film isthen removed from the top surface of the ILD by, for example, plasmaetch back or chemical mechanical planarization, to form contacts 340. Inan embodiment of the present invention, the contacts are formed from ahigh conductivity metal or metals having a low work function, such as awork function less than 4 eV. In an embodiment of the present invention,the contacts 340 are bi-layer contacts including a thin adhesion layer342 and a bulk metal 344 having a high conductivity such as illustratedin FIG. 3F. In an embodiment of the present invention, contacts 340include a thin titanium (Ti) adhesion layer of less than 150 Å and agold (Au) bulk material 344. In an embodiment of the present invention,the contacts 340 do not include any alloyed metals or silicides (i.e.,metal silicon alloy, such as nickel silicide and titanium silicide). Inthis way, the thermal budget can remain low and device performancereliability improved. Next, upper levels of metallization 350 can thenbe formed in contact with contacts 340 to electrically couple thefabricated transistor with other transistors into functional circuits.

This completes the fabrication of a field effect transistor havingsource and drain regions formed from a narrow bandgap semiconductor.

FIG. 6 illustrates a system 600 in accordance with one embodiment. Asillustrated, for the embodiment, system 600 includes computing device602 for processing data. Computing device 602 may include a motherboard604. Motherboard 604 may include in particular a processor 606, and anetworking interface 608 coupled to a bus 610. More specifically,processor 606 may comprise the device 200 or 500 that has the earlierdescribed narrow bandgap source and drain regions.

Depending on the applications, system 600 may include other components,including but are not limited to volatile and non-volatile memory, agraphics processor, a digital signal processor, a crypto processor, achipset, mass storage (such as hard disk, compact disk (CD), digitalversatile disk (DVD) and so forth), and so forth. One or more of thesecomponents may also include the earlier described narrow bandgap sourceand drain regions.

In various embodiments, system 600 may be a personal digital assistant(PDA), a mobile phone, a tablet computing device, a laptop computingdevice, a desktop computing device, a set-top box, an entertainmentcontrol unit, a digital camera, a digital video recorder, a CD player, aDVD player, or other digital device of the like.

1. A transistor comprising: a gate dielectric layer formed on asilicon-on-insulator (SOI) substrate; a gate electrode formed on thegate dielectric layer; a pair of source/drain regions on opposite sidesof the gate electrode, said pair of source/drain regions comprising asemiconductor film having a bandgap of less than 0.75 eV, wherein thesemiconductor film extends above a top surface of the gate dielectriclayer.
 2. The transistor of claim 1 wherein said semiconductor film isselected from the group consisting of indium antimonide (InSb), indiumarsenide (InAs), indium gallium arsenide (In_(1−x)Ga_(x)As(x>50%)) andindium phosphide (InP).
 3. The transistor of claim 1 wherein saidsemiconductor film is indium phosphide (InP).
 4. The transistor of claim1 further comprising a pair of non-alloyed contacts on the semiconductorfilm.
 5. The transistor of claim 4 wherein the non-alloyed contactscomprise a bi-layer of titanium/gold.
 6. The transistor of claim 1wherein the semiconductor film is doped to an n type conductivity. 7.The transistor of claim 6 wherein the semiconductor film is doped to then type conductivity with a dopant selected from the group consisting ofTe or Si.
 8. The transistor of claim 1 wherein the semiconductor filmextends above a top surface of the gate dielectric layer.
 9. Thetransistor of claim 1 wherein the SOI substrate comprises a silicon bodyhaving a top surface opposite a bottom surface formed on an insulatinglayer, and wherein the gate dielectric layer is formed on the topsurface and sidewalls of the silicon body and the gate electrode isformed on the gate dielectric layer on the top surface of the siliconbody and adjacent to the gate dielectric layer on the sidewalls of thesilicon body.
 10. The transistor of claim 1 wherein the semiconductorfilm has an electron mobility greater than 10,000 cm²V⁻¹s⁻¹.
 11. Atransistor comprising: a gate dielectric layer formed on asilicon-on-insulator (SOI) substrate; a gate electrode formed on thegate dielectric layer; a pair of source/drain regions on opposite sidesof the gate electrode, said pair of source/drain regions comprising asemiconductor film having a bandgap of less than 0.75 eV, wherein thesemiconductor film extends directly beneath the gate dielectric layer.12. The transistor of claim 11 wherein said semiconductor film isselected from the group consisting of indium antimonide (InSb), indiumarsenide (InAs), indium gallium arsenide (In_(1−x)Ga_(x)As(x>50%)) andindium phosphide (InP).
 13. The transistor of claim 11 wherein saidsemiconductor film is indium phosphide (InP).
 14. The transistor ofclaim 11 further comprising a pair of non-alloyed contacts on thesemiconductor film.
 15. The transistor of claim 14 wherein thenon-alloyed contacts comprise a bi-layer of titanium/gold.
 16. Thetransistor of claim 11 wherein the semiconductor film is doped to an ntype conductivity.
 17. The transistor of claim 16 wherein thesemiconductor film is doped to the n type conductivity with a dopantselected from the group consisting of Te or Si.
 18. The transistor ofclaim 11 wherein the SOI substrate comprises a silicon body having a topsurface opposite a bottom surface formed on an insulating layer, andwherein the gate dielectric layer is formed on the top surface andsidewalls of the silicon body and the gate electrode is formed on thegate dielectric layer on the top surface of the silicon body andadjacent to the gate dielectric layer on the sidewalls of the siliconbody.
 19. The transistor of claim 11 wherein the semiconductor film hasan electron mobility greater than 10,000 cm²V⁻¹s⁻¹.
 20. The transistorof claim 11 wherein the semiconductor film is doped to a p typeconductivity.